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One of the main tasks of forest protection is to save the forests from damage during the outbreaks of mass reproduction of forest phyllophagous insects causing their further drying out. One of these species, Gypsy moth (Lymantria dispar (L.)) belongs to the spring-summer phenological group of insects-phylophages. The defoliation of stands during the outbreaks of this species in the main part of its range usually occurs in June and early July, and after that the foliage has time to restore during the vegetative period. Whereas, on the northern boundary of this species range the heat availability is lower compared to other parts of its range, and the effect of defoliation is of considerable interest. The purpose of the study was to monitor stands after defoliation by gypsy moth and to analyze its effect on the sanitary state of the stands depending on the degree and multiplicity of defoliation on the northern border of the outbreaks (south of Sverdlovsk Region). Monitoring was carried out for 7 years after the end of the outbreaks. The changes in the sanitary state of stands and individual trees were analyzed on the basis of visual appraisal of the categories of tree state. For statistical processing, the methods of nonparametric analysis were used. The analysis showed that stands defoliation during gypsy moth outbreaks on the northern border of its range leads to a significant weakening of stands even in the most favorable, steadily fresh forest growing conditions. The results indicate that outbreaks on the northern border of the range of forest insect-phylophages and their consequences require more detailed study due to the high level of their ecological and economic significance.